1. Field of the Invention
The present invention relates to a high-side transistor driver, and more particularly to a driver circuit for driving the high-side transistor of a power converter.
2. Description of the Prior Art
Many modern-day power converters use bridge circuits to control a voltage source coupled to a load. Power supplies and motor drivers are common examples of such power converters.
A bridge circuit normally has a pair of transistors connected in series across the voltage source, with a high-side transistor connected to the voltage source and a low-side transistor connected to the ground reference. The bridge circuit includes a common node that is connected between the high-side transistor and low-side transistor. This common node is also coupled to the load.
The high-side transistor and the low-side transistor are controlled to alternately conduct. As this happens, the voltage of the common node will swing between the voltage of the voltage source and the ground reference. Thus, the voltage of the node will adjust to the level of the voltage source when the high-side transistor is turned on. This happens, because turning on the high-side transistor shifts the bridge circuit into a low impedance state. In order to fully turn on the high -side transistor, a gate driving voltage higher than that of the voltage source is required. Therefore, the voltage at the gate and at the source of the high-side transistor must be floated with respect to the ground reference.
FIG. 1 shows a prior art bridge circuit that uses a bootstrap capacitor 30 and a charge-pump diode 40 to create a floating voltage VCC for driving the gate of the high-side transistor 10. When an on/off transistor 45 is turned on, the gate of the high-side transistor 10 will be connected to the ground reference via a diode 42. This will turn off the high-side transistor 10. Once the high-side transistor 10 is turned off and a low-side transistor 20 is turned on, the floating voltage of the bootstrap capacitor 30 will be charged up by a bias voltage VB via the charge-pump diode 40. Switching off the on/off transistor 45 will propagate the floating voltage VCC via a transistor 41 to the gate of the high-side transistor 10. This will turn on the high-side transistor 10.
One drawback of this circuit is that it has high switching losses in high-voltage applications. The on/off transistor 45 requires a high voltage manufacturing process to be suitable for high-voltage source applications (200 volts or more). Such high-voltage transistors typically have a large parasitic capacitor, which will increase the rising-time and slow down the switching signal. This will result in high switching losses from the high-side transistor. Therefore, this prior-art bridge circuit is inadequate for high-voltage and high-speed applications.
Many recently developed bridge circuit designs include methods of generating a suitable gate-voltage for the high-side transistor. Some well known prior-art inventions include U.S. Pat. No. 5,381,044 (Zisa, Belluso, Paparo), U.S. Pat. No. 5,638,025 (Johnson), and U.S. Pat. No. 5,672,992 (Nadd). These prior-art bridge circuits share the same drawbacks as the circuit shown in FIG. 1. The on/off transistors of these prior-art inventions cause high switching losses in high-voltage applications.
To overcome some of these objections, a prior art bridge circuit using a boost converter technique has been introduced in U.S. Pat. No. 6,344,959 (Milazzo). However, this technique uses a voltage doubling circuit that requires an additional switching element as well as other circuitry, thereby adding to the cost and complexity of the driving circuit. Moreover, high frequency charging and discharging of the voltage doubling capacitor in the charge pump will result in severe noise being generated at the voltage source terminal and the ground reference terminal.
The objective of the present invention is to overcome the drawbacks of prior art bridge circuits and to provide a high-side transistor driver that is suitable for high-voltage and high-speed applications.